Process of producing oxides of platinum and palladium



United States Patent Ofifice 3,357,904 Patented Dec. 12, 1967 3,357,904 PROCESS OF PRODUCING OXIDES OF PLATINUM AND PALLADIUM Raymond Steele, West Chester, Pa., assignor to J. Bishop 1 .& Co. Platinum Works, Malvern, Pa, a corporation of Pennsylvania No Drawing. Filed Mar. 3, 1967, Ser. No. 620,282

10 Claims. (Cl. 204-61) ABSTRACT OF THE DISCLOSURE Description of invention The present application is a continuation-in-part of my copending application, Ser. No. 370,690, filed May 27, 1964 now abandoned for Process of Producing Oxides of Platinum and Palladium, and Platinum and Palladium Oxides.

The present invention relates to processes for producing oxides of platinum and palladium or mixed oxides based on platinum or palladium.

A purpose of the invention is to produce an oxide of platinum or palladium or mixed oxides more reliably and economically, with high activity.

A further purpose is to avoid tying up a large amount of platinum or'palladium or platinum or palladium alloys in processing to make platinum or palladium oxide or mixed oxides.

A further purpose is to obtain a higher yield of platinum or palladium oxide or mixed oxides.

A further purpose is to cut down the processing time of platinum or palladium to produce oxide.

A further purpose is to avoid danger to personnel by i eliminating risk of platinosis in making platinum oxide. A further purpose is to reduce the risk of metal loss in making platinum oxide, palladium oxide or mixed oxides. l

A furtherpurpose is to avoid the necessity to dissolve platinum or palladium prior to the actual production of the oxide and avoid the necessity to process mother liquors resulting therefrom.

A further purpose is to eliminate the need to recover platinum or palladium from spent liquors in making the oxide.

A further purpose is to avoid the danger from acid fumes incident to dissolving platinum in aqua regia.

A further purpose is to produce platinum oxide, palladium oxide or mixed oxides of superior catalytic activity by electrolyzing the pure metal or an alloy as an anode in a molten bath of alkali metal nitrate in the presence of from about 0.5% to 10% by weight of alkali metal halide, preferably the chloride.

Further purposes appear in the specification and in the claims.

Platinum oxide, PtO or PtO H O, is used extensively as a catalyst, particularly for the liquid phase hydrogenation of organic compounds. Palladium oxide is also used for similar purposes.

The usefulness of platinum oxide for this purpose was discovered by Dr. Roger Adams at the University of Il linois in 1922. He prepared platinum oxide in very small amounts by fusion of chloroplatinic acid and sodium nitrate. He found that when this oxide was added to a solution of a compound to be hydrogenated, it was reduced to platinum black at the outset of the hydrogenation, and the freshly formed platinum black was far more effective in accelerating the reaction than the best platinum blacks previously obtainable. This platinum oxide is often referred to as Adams Catalyst.

Platinum oxide is prepared commercially through the reaction of ammonium or potassium chloroplatinate and molten sodium nitrate. It is first necessary to convert platinum metal to the chloroplatinate by a rather lengthy procedure. Platinum plates are first rolled into thin sheets of a thickness of about 0.010", and the sheets are cut into strips and then dissolved in aqua regia. When solution is complete and the remaining nitric acid has been ex pelled from the solution, the platinum is precipitated as ammonium or potassium chloroplatinate, (NH PtCl or K PtCl This chloroplatinate is filtered, washed and thoroughly dried. The platinum which remains in the mother liquor is collected by reduction with iron and this must be purified before it can be again used.

The dry chloroplatinate salt is aded to a molten bath of sodium nitrate which at this state is held at a temperature of 350 to 400 C. Whenjhe entire charge of chloroplatinate salt has been added, the temperature of the melt is raised to about 600 C. and the molten salt mixture is then cast into a thin sheet and allowed to solidify as a cake. The resulting cake is leached with hot water and the platinum oxide is separated by filtration. The filtrate must be treated for platinum recovery.

Alternatively the melt may be poured directly into Well stirred water. After a further treatment with hot, dilute nitric acid, the oxide is again filtered and is then dried. The product assays in the range of to platinum.

This prior art process has a number of serious disadvantages. It ties up the platinum which is in process for a lengthy period, and thus is costly from an investment standpoint. A substantial amount of platinum is diverted into spent liquors. Some real loss of platinum occurs, as all platinum processing particularly in solutions is accompanied by a real loss whose magnitude depends upon the number and complexity of the processing steps.

Severely corrosive fumes are generated when the platinum is dissolved in aqua regia and once again when the chloroplatinate reacts with sodium nitrate and this constitutes a hazard for personnel and a maintenance problem for equipment. Operating personnel are subjected by this prior art process to risk of platinosis, which is a respiratory or a skin affliction which may result from their handling of dry chloroplatinate salts.

Another disadvantage of the prior art is that the activity of the platinum oxide obtained is rather mediocre. For example, four 0.10 gram samples of commercial platinum oxides from three different sources, designated as A, B and C, were tested under identical conditions in the hyd'rogenation of substances listed in the table below with the following results:

RESULTS OF CATALYTIC ACTIVITY TESTS ON COMMER- CIAL PLATINUl/l' OXIDES Source of Oxide (min) Time Required to Hydrogenate 0.1 Mole Maleic Acid- Benzaldehyde In previous experiments, I found that more active platinumoxides could be made by adding other compounds of platinum to molten sodium nitrate in the process of the prior art. For example, when potassium chloroplatinite, K PtCld, was used, a platinum oxide was produced which made it possible to hydrogenate maleic acid in nine minutes. When platinum tetrachloride, Ptcl was dissolved in sodium nitrate solution, and the mixture was evaporated to dryness and the dry crystals added to the molten sodium nitrate, a platinum oxide was obtained which made possible hydrogenation of maleic acid in eight minutes. These catalysts are very active. However, the salts used in making them are expensive and make the platinum oxide so costly that it cannot be used extensively.

I have discovered that it is possible to produce platinu'm' oxide, palladium oxide and mixtures of platinum or palladium oxide, with oxides of other metals by electrolyzing the metal or alloy anodically ina molten bath of alkali metal nitrate containing some alkali metal chloride.

In the broadest aspects of the invention, the nitrate may be any alkali metal nitrate, such as sodium, postassium, or lithium nitrate, or a mixture thereof. It is decidedly preferable to use sodium nitrate alone or with a minor addition (not exceeding 20% by weight) of another alkali metal nitrate, such as potassium or lithiumnitrate or a mixture thereof.

The alkali metal chloride should preferably be lithium chloride althoughit can be sodium' or potassium chloride.

Relatively minute quantities of alkali metal halide are all that are required. For example, in abath of 650 grams of sodium nitrate 13' grams of lithium chloride gave good results under the conditions referred to in the example below. 0.5% of alkali metal chloride is enough.

It will be evident that the temperature must be high enough to maintain the bath molten and this will, of

course, depend on the particular nitrate and halide involved. With sodium nitrate as the major component and a small amount of lithium chloride included, good results can be obtained by electrolyzing at a temperature of 340 C. to 520 C. The temperature should not be high enough to" cause excessive difliculty withfuming or decomposition of the bath.

In the process of the invention, there is no need to place the platinum or palladium in solution by a prior step. The solution of the platinum or palladium and its conversion to platinum or palladium oxide are practically simultaneous.

The product of the invention is an active catalyst as explained below.

The disadvantages of prior art processes are largely eliminated.

The anode can be a heavy platinum or palladium or alloy plate, thus avoiding the necessity to roll into thin sheets and cut into strips. The unreacted portion of the anode is available immediately for remelting with other platinum or palladium and does not have to be recovered from solution.

Theyield of platinum oxide or palladium oxide is substantially since the mother liquor in the process of the invention is free from platinum or palladium.

In the process of the invention, unlike the prior art, the platinum oxide or palladium oxide is commercially available within a few days after the electrolysis takes place.

Unlike the prior art processes, no obnoxious fumes are produced and the only caution which needs to be exercised is to avoid contact with the fine spray entrained in the gases liberated at the electrodes. The process of the invention eliminates the risk to personnel of platinosis. The process of the invention also greatly reducesthe risk of metal loss.

In the several examples listed inthe table below, a bath consisting of 650 grams of sodium nitrate was melted in a 400 cc. beaker (a Vycor beaker was used) in an electrically heated crucible furnace. 13 grams of lithium chloride was dissolved in this melt.

The cathode consisted of 9 nickel wires of 0.05" diameter. The anodewasa strip of platinum, platinum alloy or palladium 1%" wide, in thicknesses between 0.040" and 0.125", and extending into the molten bath to a depth of 2.5". The spacing between the anode and the cathode was 1.5". The electrolysis occurred under a voltage of from 4 to 4.5 volts DC. The molten salt bath was mechanically stirred with a stainless steel propellor rotating at a shaft speed of 200 r.p.m.

The temperature of the melt during electrolysis was maintained at 360 C. within plus or minus 10' C. At the conclusion of the electrolysis the melt was heated to 600 C. and held at this temperature for five minutes. The melt was then cast into a stainless steel tray. The solidified mass was broken into lumps and then leached with hot water from which the platinum oxide settled out. The platinum oxide was filtered; stirred with hot 10% nitric acid, refiltered and dried at 180 C. Themot-her liquor was rejected as it contained no platinum;

In Examples A to I referred to below, the same procedure was followed throughout except that in Example D the step of heating the melt to 600 C. was omitted and themelt was cast directly from the electrolysis bath, at a temperature of 360 CL In Example E, instead of using 13 grams of lithium chloride in- 650 grams of sodium nitrate, the alkali metal halide used was 18 grams of sodium chloride.

In Examples A to E the anode was pure platinum of a purity of better than 99.9%. It is notedthat all of the platinum oxides gave favorable results in hydrogenating. maleie acid and benzaldehyde as compared with commercially available catalysts previously mentioned.

In Example F the anode was 95% platinum and 5% rhodium, and it gave a very favorablecatalyst. In Example G- the anode was 95% platinum and 5% iridium. It also gave a good catalyst. In Example H the anode was 95% platinum and 5% ruthenium, and the results were very favorable.

In Example I the anode was pure palladium and a catalytically effective palladium oxide was produced.

It will be noted that in Examples C and E high current densities were used and in Example A a very low cur-- rent density was used. Active catalysts were produced over a wide range of current density. My experiments indicate that while a wide temperature range can be used, a low temperature such as 360 C. is preferred. I also prefer to use about 50' parts of nitrate to one part of halide.

I have also obtained good results using as rnuch as 1 0% by weight of lithium chloride in molten sodium nitrate at electrolysis temperatures ranging from 340 C. to 520 C. I have also obtained good results in melts to which an alkali metal hydroxide such as sodium hydroxide has been added in small increments periodically during electrolysis inproportions from 0.1 to 1%. Alkali metal hydroxide addition tends to lower the density of the platinum or palladium oxide and this is desirable in some cases.

1 In preparation D, the step of heating the melt to 600 0. was omitted and the melt was cast immediately following electrolysis.

2 In preparation E, 18 grams of sodium chloride was used instead of the 13 grams of lithium chloride used in the other preparations.

Example J A melt was made of 574-grams of sodium nitrate and 5 grams of lithium chloride in a 400 cc. Vycor beaker. The temperature of the melt was adjusted to 360 C. and electrolysis was carried out for 326 minutes at 2 amperes. The anode was a 1" wide platinum plate submerged in the melt for 2" of its length. After electrolysis, the melt was cast, cooled, leached with Water, filtered, leached further with hot dilute nitric acid, refiltered and dried. 26.06 grams of platinum were converted to the oxide. 0.1 gram of the oxide catalyzed the hydrogenation of 0.1 mole of maleic acid in 8.5 minutes. The lithium chloride comprises 0.86% of the salt mixture in this example.

Example K A melt was made of 646.5 grams of sodium nitrate and 3.5 grams of lithium chloride in a 400 cc. Vycor beaker. The temperature of the melt was adjusted at 360 C. and electrolysis was carried out for 300 minutes at 3 amperes. The anode was a 1%" wide palladium plate submerged in the melt for 2 /2" of its length. Following electrolysis, the melt was cast, cooled, leached with water, filtered, leached further with hot dilute nitric acid, refiltered and dried. 15.12 grams of palladium were converted to the oxide. 0.1 gram of this oxide catalyzed the hydrogenation of 0.1 mole of maleic acid in 19 minutes. The lithium chloride comprises 0.47% of the salt mixture in this example.

I have employed containers of nickel and of stainless steel with success.

I have made successful runs without agitation and also have agitated by horizontal reciprocal motion imparted by an electrode rocker.

Good results have been obtained using a nickel sheet as well as nickel wire, or a nickel container as the cathode.

It will be evident that the invention is applicable to produce either platinum, palladium or mixtures of platinum and palladium oxides from these metals or their platinum, palladium or platinum-palladium base alloys including other noble metals, or any harmless base metal, such as nickel, in melts of alkali metal nitrate with an alkali metal halide as previously described. The platinum or palladium or both should be at least 50% of the alloy by weight.

One of the virtues of the invention is that it makes it possible to produce highly effective oxide catalysts for hydrogenation or other similar processes.

The oxides of this invention can be converted to useful blacks and powders by suitable reduction. They are useful also in making platinum compounds which may be obtained by reaction with platinum oxide.

In view of my invention and disclosure, variations and modifications to meet individual whim or particular need will doubtless become evident to others skilled in the art to obtain all or part of the benefits of my invention without copying the process and composition shown, and I, therefore, claim all such insofar as they fall within the reasonable spirit and scope of my claims.

Having thus described my invention what I claim as new and desire to secure by Letters Patent is:

1. A process of producing oxides of platinum or palladium, which comprises maintaining a molten bath of a nitrate of an alkali metal in the presence of from about /2% to 10% by Weight of a chloride of an alkali metal, and electrolyzing said molten bath with a noble metal containing anode of the class consisting of platinum, palladium, alloys of platinum and palladium, platinum base alloys with other metals, palladium base alloys with other metals and platinum-palladium base alloys with other metals and recovering said oxide from said molten bath.

2. A process of claim 1, which comprises maintaining the molten bath at a temperature in the range between 340 and 520 C.

3. A process of claim 1, in which the alkali metal nitrate is sodium nitrate.

4. A process of claim 1, in which the alkali metal chloride is lithium chloride.

5. A process of claim 1, which comprises adding to the bath an alkali metal hydroxide.

6. A process of claim 1, in which the anode essentially consists of platinum.

7. A process of claim 1, in which the anode essentially consists of palladium.

8. A process of claim 1, in which the anode essentially consists of platinum and rhodium.

9. A process of claim 1, in which the anode essentially consists of platinum and iridium.

10. A process of claim 1, in which the anode essentially consists of platinum and ruthenium.

References Cited UNITED STATES PATENTS 3/1894 Darling 20468 1/ 1909 Carrier 2046l OTHER REFERENCES ROBERT K. MIHALEK, Primary Examiner.

D. R. VALENTINE, Assistant Examiner. 

1. A PROCESS OF PRODUCTING OXIDES OF PLATINUM OR PALLADIUM, WHICH COMPRISES MAINTAINING A MOLTEN BATH OF A NITRATE OF AN ALKALI METAL IN THE PRESENCE OF FROM ABOUT 1/2% TO 10% BY WEIGHT OF A CHLORIDE OF AN ALKALI METAL, AND ELECTROLYZING SAID MOLTEN BATH WITH A NOBLE METAL CONTAINING ANODE OF THE CLASS CONSISTING OF PLATINUM, PALLADIUM, ALLOYS OF PLATINUM AND PALLADIUM, PLATINUM BASE ALLOYS WITH OTHER METALS,PALLADIUM BASE ALOYS WITH OTHER METALS AND PLATINUM-PALLADIUM BASE ALLOYS WITH OTHER METALS AND RECOVERING SAID OXIDE FROM SAID MOLTEN BATH. 